Detecting transducer position precisely with a reusable sensor assembly

ABSTRACT

A method and apparatus for transducer position detection uses inert contact surfaces to engage an actuator temporarily. The inert contact surfaces are part of an apparatus that also includes an optical sensor such as a retroreflector or diffraction grating. The assembly is thus configured to permit multiple uses while exerting a near-zero net torque about the length of the arm that supports the transducer. This is useful where there is a temporary need for especially precise position control, such as when servowriting a data storage media surface.

RELATED APPLICATIONS

[0001] This application claims priority of U.S. provisional applicationSerial No. 60/314,039 filed Aug. 22, 2001.

FIELD OF THE INVENTION

[0002] This application relates generally to position sensing and moreparticularly to temporarily enhancing the accuracy of positionmeasurements.

BACKGROUND OF THE INVENTION

[0003] Disc drives are data storage devices that store digital data inmagnetic form on a rotating disc. Modern disc drives comprise one ormore rigid information storage discs that are coated with a magnetizablemedium and mounted on the hub of a spindle motor for rotation at aconstant high speed. Information is stored on the discs in a pluralityof concentric circular tracks typically by an array of transducersmounted to a radial actuator for movement of the heads relative to thediscs. During a data write operation sequential data is written onto thedisc track, and during a read operation the head senses the datapreviously written onto the disc track and transfers the information toan external environment. Important to both of these operations is theaccurate and efficient positioning of the head relative to the center ofthe desired track on the disc. Head positioning within a desired trackis dependent on head-positioning servo patterns, i.e., a pattern of databits recorded on the disc surface and used to maintain optimum trackspacing and sector timing. Servo patterns or information can be locatedbetween the data sectors on each track of a disc (“embedded servo”), oron only one surface of one of the discs within the disc drive(“dedicated servo”). Regardless of whether a manufacturer uses“embedded” or “dedicated” servos, the servo patterns are typicallyrecorded on a target disc during the manufacturing process of the discdrive.

[0004] Recent efforts within the disc drive industry have focused ondeveloping cost-effective disc drives capable of storing more data ontoexisting or smaller-sized discs. One potential way of increasing datastorage on a disc surface is to increase the recording density of themagnetizable medium by increasing the track density (i.e., the number oftracks per inch). Increased track density requires more closely-spaced,narrow tracks and therefore enhanced accuracy in the recording ofservo-patterns onto the target disc surface. This increased accuracyrequires that servo-track recording be accomplished within the increasedtolerances, while remaining cost effective.

[0005] Servo patterns are typically recorded on the magnetizable mediumof a target disc by a servo-track writer (“STW”) assembly during themanufacture of the disc drive. One conventional STW assembly recordsservo pattern on the discs following assembly of the disc drive. In thisembodiment, the STW assembly attaches directly to a disc drive having adisc pack where the mounted discs on the disc pack have not beenpre-recorded with servo pattern. The STW does not use any heads of itsown to write servo information onto the data surfaces, but uses thedrive's own read/write heads to record the requisite servo pattern tomounted discs.

[0006] To facilitate this process, especially in light of trackdensities now exceeding 100,000 tracks per inch, more accuratepositioning is required at a much lower cost. Such positioning requireshigh quality sensor instruments including an optical sensor. To makesuch instrumentation cost effective, what is needed is an effectivemechanism for key components to be placed temporarily, then removed andreused. The present invention provides a solution to this and otherproblems, and offers other advantages over the prior art.

SUMMARY OF THE INVENTION

[0007] The present invention is a method and apparatus for detectingtransducer position precisely using an assembly with at least one inertcontact surface for engaging an actuator temporarily. The inert contactsurface(s) are part of an assembly that includes an optical sensor forvery precise position sensing.

[0008] A first preferred embodiment is a device for sensing a positionof a transducer of an actuator having an arm with a transducer at itsdistal end. The device includes a circuit configured for detecting aposition of the transducer based on a position-indicative measurement.The device also includes engagement means for supporting the positionsensor in a fixed position relative to the actuator while exerting anear-zero net torque about the arm axis.

[0009] A second preferred embodiment is a method including a step ofbringing the inert contact surface(s) to bear against the actuator. Italso includes moving the actuator while the reusable assembly in contacttherewith exerts a near-zero net torque about the axis, and subsequentlyseparating the inert contact surface(s) from the actuator.

[0010] Additional features and benefits will become apparent uponreviewing the following figures and their accompanying detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a data storage device constructed in accordance witha preferred embodiment of the present invention.

[0012]FIG. 2 shows a more magnified view of sensor assembly of FIG. 1 asit approaches the top surface of the body of the actuator of FIG. 1.

[0013]FIG. 3 shows the sensor assembly and actuator of FIG. 2 clampedtogether.

[0014]FIG. 4 shows a side view of the configuration of FIG. 3, furthermagnified and further defining the clamped engagement.

[0015]FIG. 5 shows a detailed flowchart of a method of the presentinvention.

[0016]FIG. 6 is a partially exploded view showing how another reusableapparatus of the present invention engages a bottom surface of anactuator of another disc drive.

[0017]FIG. 7 shows a top view of the disc drive of FIG. 6.

DETAILED DESCRIPTION

[0018] Although the examples below show more than enough detail to allowthose skilled in the art to practice the present invention, subjectmatter regarded as the invention is broader than any single examplebelow. The scope of the present invention is distinctly defined,however, in the claims at the end of this document.

[0019] Numerous aspects of data storage device technology that are not apart of the present invention (or are well known in the art) are omittedfor brevity, avoiding needless distractions from the essence of thepresent invention. For example, this document does not include muchdetail about using properly placed optical elements for measuringposition. Neither does it include specific methods for seeking or theuse of zero acceleration path (ZAP) correction factors. Specificmaterials for constructing components described herein are likewisetypically omitted, being a simple matter of design choice.

[0020] Definitions and clarifications of certain terms are provided inconjunction with the descriptions below, all consistent with commonusage in the art but some described with greater specificity. Forexample, an “arm axis” is a line passing through the arm and generallycorresponding to its longest dimension. An element that exerts a“near-zero” net torque is one that exerts a torque of at most about 0.04Newton-meters. (Applicant has ascertained that larger torques inducesignificant errors when exerted upon an actuator about its arm axes.)

[0021] Turning now to FIG. 1, there is shown a data storage device 100constructed in accordance with a preferred embodiment of the presentinvention. Device 100 is a disc drive including base 102 to whichvarious components are mounted. Top cover 123 cooperates with base 102conventionally to form a sealed chamber. The components include aspindle motor which rotates data storage discs 110 at several thousandrevolutions per minute. Information is written to and read from tracks112 on discs 110 through the use of an actuator assembly 161, whichrotates during a seek operation about a bearing shaft assembly 130positioned adjacent discs 110. Actuator assembly 161 includes aplurality of actuator arms 162 which extend above and below each disc110, with one or more flexures extending from each of the actuator arms.Each arm has a corresponding axis 142 along its length. Mounted at thedistal end of each of the flexures is a transducer head 134 whichincludes an air-bearing slider enabling transducer head 134 to fly inclose proximity above the corresponding surface of associated disc 110.

[0022] Servo and user data travels through transducer head 134 and flexcable 180 to control circuitry on controller board 106. Flex cable 180maintains an electrical connection by flexing as transducer heads 134traverse tracks 112 along their respective radial paths 138. By“radial,” it is meant that path 138 is substantially aligned with aradius of the disc(s) 110, although their directions may be offset froma perfectly radial direction by up to about 20 degrees due to head skew,as is understood in the disc drive industry.

[0023] During a seek operation, the overall track position of transducerheads 134 is controlled through the use of a voice coil motor (VCM),which typically includes a coil 122 fixedly attached to actuatorassembly 161, as well as one or more permanent magnets 120 whichestablish a magnetic field in which coil 122 is immersed. The controlledapplication of current to coil 122 causes magnetic interaction betweenpermanent magnets 120 and coil 122 so that coil 122 moves. As coil 122moves, actuator assembly 161 pivots about bearing shaft assembly 130 andtransducer heads 134 are caused to move across the surfaces of discs 161between the inner diameter and outer diameter of the disc(s) 161. Finecontrol of the position of head 134 is optionally made with amicroactuator (not shown) that operates between the head 134 and theactuator arm.

[0024] Arcuate slots 128,129 are provided in top cover 123 so thatsensor assembly 190 can be moved (down) into contact with the topsurface of actuator assembly 161. Slots 128,129 permit sensor assembly190 to remain clamped to actuator assembly 161 as it rotates across itsentire range of motion (i.e. corresponding to path 138). Diffractiongrating 125 affixed onto sensor assembly 190 is used to generate anextremely accurate position indication of head 134 as head 134 writesservo marks onto discs 110. Then, sensor assembly 190 disengages fromactuator assembly 161, and slots 128,129 are covered (e.g. by tape).

[0025]FIG. 2 shows a more magnified view of sensor assembly 190 as itapproaches the top surface 211 of the body 261 of actuator 161. (Cover123 and slots 128,129 are not shown here, for clarity.) This view ofsensor assembly 190 shows two clamping rods 271,272 and two precisionlocator rods 281,282. As the sensor assembly 190 is lowered into place,the clamping rods 271,272 enter corresponding recesses 231,232 formed inthe top surface 211 of actuator body 261. Recesses 231,232 are axiallysymmetric, formed by a tapered bore as shown in FIG. 4. As the sensorassembly is clamped into place, locator rods 281,282,283 simultaneouslycome into contact with three positions 241,242,243 on the flat portionof the top surface 211. (Note that the rear locator rod 283 is obscuredbehind clamping rod 272, but is shown in FIG. 4.)

[0026]FIG. 3 shows sensor assembly 190 affixed onto actuator assembly161. The blunt ends of all three locator rods 281,282,283 contact topsurface 211, effectively preventing sensor assembly 190 from tilting orsliding downward under normal (servowrite) operating conditions. FIG. 3also identifies a side view 400 of sensor assembly 190 engaging actuatorassembly 161.

[0027]FIG. 4 shows side view 400, further magnified and further definingthe engagement. Prongs 291 of clamping rods 271,272 each engage atapered portion of corresponding recesses 231,232, effectivelypreventing sensor assembly 190 from sliding upward (i.e. under normaloperating conditions). Shoulder 292 around the entire circumference ofclamping rod 271 provides for a friction fit with an upper portion ofrecess 231, effectively preventing lateral movement of rod 271 relativeto actuator assembly 161. A similar shoulder around opposing sides ofclamping rod 272 effectively prevents sensor assembly 190 from pivotingabout recess 231. Note that the shoulder of clamping rod 272 is notvisible in FIG. 4 because it does not extend to the sides of rod 272that are nearest and farthest from rod 271. The absence of a shoulder ontwo sides of rod 272 permits effective engagement despite thermal andmanufacturing variations in the relatively large distance between theclamping rods 271,272.

[0028] All of the rods 271,272,273,281,282,283 of sensor assembly 190are made of a resilient metal, preferably stainless steel. All of thesurfaces of sensor assembly 190 that contact actuator assembly 161 areinert (i.e. not reliant on adhesives or prone to leaving deposits ofproblematic impurities). To further guard against sensor assemblybecoming dislodged during a shock, sensor assembly 190 desirably has arotational inertia less than 1% of that of actuator assembly 161 abouttheir mutual axis of rotation 250.

[0029]FIG. 5 shows a method 500 of the present invention comprisingsteps 505 through 575. A reusable sensor assembly is constructed byaffixing a body to a retroreflector and to a piston that can slide alongand rotate within a cylindrical sleeve 510. Suitable high precisionpistons and sleeves can be purchased from Airpot Corporation in Norwalk,Conn. As literature from that company shows, the piston and sleeve actas a U-joint, permitting two rotary elements to rotate together despiteimperfect axial alignment. By pressurizing the chamber between thepiston and sleeve, moreover, the sensor assembly body can be urgedagainst the actuator body.

[0030] After affixing the sleeve to the STW chassis 515 and constructingthe HDA 520, the airpot is aligned with the actuator 525. With the HDAactuator at a home position 535, the actuator is brought into contactwith the sensor assembly body 535. This is analagous to the couplingstep illustrated in FIGS. 2&3. As explained above, the airpot ispressurized to urge inert surfaces of the sensor assembly against theactuator 540. This permits the sensor assembly to remain against theactuator without clamping and while exerting a near-zero torque on theactuator arms (i.e. about their respective axes). After calibrating thesensor 545, the sensor is used to control servo track writing 555 overthe rest of the HDA (which my have several data surfaces). The airpotpressure is released to permit easy disengagement 560. The reusablesensor assembly is then re-used 570 many times before a re-build isnecessary.

[0031]FIG. 6 is a partially exploded view showing how another reusableapparatus 610 of the present invention engages a bottom surface of anactuator 661 of another disc drive 600. Actuator 661 has five arms asshown, each of which can support one or two heads. Inert contactsurfaces on the tops of three posts 621,622,623 protrude upward fromcontact element 620. Retroreflector 625 is glued to contact element 620(as shown), as is ring 630 (shown not in contact). Hollow element 640 isrigidly supported on a chassis of a servo track writer (not shown). Whenactuator 661 is properly aligned with contact element 620 (as shown),the two round post ends enter recesses in actuator 661, as shown. Thehousing of disc drive 600 is held in a fixed position relative to theservo track writer and hollow element 640. This substantially limitsactuator 661 from all but one degree of motion (i.e. rotating about itsaxis). A gas such as air is pumped into inlet 649, increasing pressureat the eight outlets 641 until ring 630 separates from hollow element640. Groove 631 equalizes pressure about the axis of rotation 692,tending to create a uniform flow that causes ring 630 to float.

[0032] For positive engagement, a first recess 711 in actuator uses aconical taper which self aligns with post 622. A second recess 712 usesa groove aligned with post 623 and generally toward post 622 so as toself-align despite some spatial variation between posts 622 and 623.Post 621 has a flat bottom that simply comes to rest on a flat portionof actuator 661 (not self-aligning, except vertically).

[0033] During operation, circuit 629 activates emitter 624, which emitslight into retroreflector 625 with a known direction. Depending on theposition of retroreflector 625, it reflects the light to a differentposition on receptor 626. Receptor 626 thereby generates a signal 628indicative of that position back to circuit 629. Further details forusing optical elements for measuring position are taught in U.S. Pat.Nos. 5,227,625; 5,442,172; and 5,796,542.

[0034]FIG. 7 shows a top view of disc drive 600, showing the horizontalpositions of some key elements. Similar to that of FIG. 1, a disc stackrotates about axis 691. Actuator 661 rotates about axis 692. Withreusable apparatus 610 held in alignment and contact with actuator 661as indicated in FIG. 6, retroreflector 625 is held in rigid contact withactuator 661.

[0035] Monitoring the position of retroreflector 625 as actuator 661moves provides an extremely accurate indication of the position ofactuator 661. Yet this configuration does not twist or rock actuator 661relative to any of the arm axes 642 (shown in FIG. 6) substantiallyenough to introduce any errors. Moreover, apparatus 610 and hollowelement 640 can both be reused on hundreds or thousands of disc drives.

[0036] Alternatively characterized, a first embodiment of the presentinvention is an apparatus (such as 190,610) for detecting a position ofa transducer (such as 134) supported by an arm (such as 162,662) of anactuator. The arm has an arm axis (such as 142,642) passing therethroughsubstantially along a length of the arm. The apparatus includes acircuit (such as 629) configured for detecting a position of thetransducer based on a position-indicative measurement (such as 628) ofan optical position sensor (such as 125,625). The apparatus alsoincludes engagement means (such as 271,281,610,621) for supporting theposition sensor in a fixed position relative to the actuator whileexerting a near-zero net torque about the arm axis (such as 142,642).

[0037] In a second embodiment, the engagement means is a single-piecerigid element (such as 190) that bears against the actuator in adirection substantially aligned with an axis of rotation (such as 250).To further maintain rigidity, the rigid element operates without makingany contact with any protrusion (such as arm 162) of the actuator. Also,the engagement means is configured to engage only one end surface (suchas 211) of only one HDA at a time (such as by method 500).

[0038] In a third embodiment, the engagement means includes a gasbearing that can exert a force >1 Newton to compress the actuator alongthe spindle axis (such as 192,692) via several inert contact surfaces(such as those of 621) distributed about the spindle axis. At least oneof the inert contact surfaces engages a tapered portion of the interiorof a hole (such as 711) in the actuator. Each of the respective forceshas a respective component along the spindle axis greater than 0.1Newtons.

[0039] In a fourth embodiment, the engagement means includes an elasticdeformation element (such as 291) clamping the position sensor in thefixed position relative to the arm. The engagement means consists ofseveral inert contact surfaces that can be clamped simultaneously (asshown in FIGS. 3& 4) each against a respective predetermined portion ofthe actuator.

[0040] A fifth embodiment of the present invention is a method fortransducer position detection method including steps for providing (a) areusable assembly comprising a position sensor and at least one inertcontact surface and (b) an actuator having an arm supporting atransducer (such as by steps 510 and 520). An arm axis (such as 642)passes through the arm, substantially along its length. The contactsurface(s) are brought to bear against the first actuator (such as bysteps 535,540). When the actuator moves (or is moved such as by aservowriter arm outside the drive), the reusable assembly remains infixed relation to it while exerting a near-zero net torque about the armaxis. The net torque is more preferably less than 0.02 Newton-meters andmost preferably less than 0.01 Newton-meters. This permits accurateposition detection with reduced errors and with a readily detachable,reusable assembly.

[0041] A sixth embodiment of the present invention is a method includingconstructing the reusable assembly as a piston that can slide along androtate within a round sleeve (such as y step 510). The method alsoincludes adjusting a pressure between the sleeve and the piston so as tocontrol a force exerted by the reusable assembly upon the actuator (suchas by step 540). In this way, the force is made substantiallyindependent of any rotation of the piston relative to the sleeve. Afterreleasing the adjusted pressure, a very small separation force (<0.01Newtons) is used to separate the reusable assembly from the actuator(such as by step 560).

[0042] A seventh embodiment of the present invention is a servo writingmethod including calibration steps. The actuator is positioned againstmechanical stops at each extreme of its motions, taking a reading of theposition sensor at each (such as by step 545). These readings are usedto generate a calibration multiplier (such as W) that is used withmeasurements from the sensor to derive control values. The controlvalues help maintain the selected arm at a desired position during aservo write operation (such as by step 555).

[0043] An eighth embodiment of the present invention includes a step ofbringing the actuator to a home position at which the transducer is notadjacent any stored user data (such as by step 535). This permits theinert surfaces to engage the actuator at precise, predetermined areas ofthe actuator (such as 231,241).

[0044] All of the structures and methods described above will beunderstood to one of ordinary skill in the art, and would enable thepractice of the present invention without undue experimentation. It isto be understood that even though numerous characteristics andadvantages of various embodiments of the present invention have been setforth in the foregoing description, together with details of thestructure and function of various embodiments of the invention, thisdisclosure is illustrative only. Changes may be made in the details,especially in matters of structure and arrangement of parts within theprinciples of the present invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed. For example, the inert surfaces can be on the insides ofclamps configured to engage protrusions on the actuator, rather than onthe ends of posts. In addition, although the preferred embodimentsdescribed herein are largely directed to disc drives, it will beappreciated by those skilled in the art that many teachings of thepresent invention can be applied to other applications where finepositioning is needed temporarily, without departing from the scope andspirit of the present invention.

What is claimed is:
 1. A transducer position detection method comprisingsteps of: (a) providing a position sensor having a fixed spatialrelation with at least one inert contact surface; (b) providing a firstactuator having a first arm supporting a first transducer having aposition, the first arm having a first axis passing therethroughsubstantially along a length of the first arm; (c) bringing the inertcontact surface(s) to bear against the first actuator; (d) moving thefirst actuator while the inert surface(s) in contact therewith exert(s)a near-zero net torque about the axis; (e) while the first actuatormoves in the moving step (d), detecting the position of the firsttransducer based on a position-indicative measurement of the positionsensor; and (f) separating the inert contact surface(s) from the firstactuator.
 2. The method of claim 1, in which the moving step (d)includes steps comprising: (d1) constructing a reusable assemblyincluding the position sensor and a piston that can slide along androtate within a round sleeve; and (d2) adjusting a pressure between thesleeve and the piston so as to control a force exerted by the reusableassembly upon the actuator, the force being substantially independent ofany rotation of the piston relative to the sleeve.
 3. The method ofclaim 1, in which the moving step (d) includes steps comprising: (d1)positioning the actuator against a first mechanical stop and measuringan outermost resulting position of the position sensor; (d2) positioningthe actuator against an outer mechanical stop and measuring an innermostresulting position of the position sensor; and (d3) calibrating thesensor based on the measured outermost and innermost positions of steps(d1) and (d2).
 4. The method of claim 1, in which the step of separating(f) is performed by applying a separation force less than 0.01 Newtons.5. The method of claim 1, in which the moving step (d) is performedwhile the inert surface(s) exert(s) a net torque less than 0.02Newton-meters about the axis.
 6. The method of claim 1, in which themoving step (d) is performed while the inert surface(s) exert(s) a nettorque less than 0.01 Newton-meters about the axis.
 7. The method ofclaim 1, in which the bringing to bear step (c) includes a step (c1) ofaligning a reusable assembly comprising the inert contact surface withan axis of rotation of the actuator.
 8. The method of claim 1, in whichthe bringing to bear step (c) includes a step (c1) of clamping the inertsurface to the actuator.
 9. The method of claim 1, in which the bringingto bear step (c) includes an initial step (c1) of bringing the firstactuator to a home position at which the first transducer is notadjacent any stored user data.
 10. The method of claim 1, in which theproviding step (a) comprises a step of gluing the position sensor to abody comprising the inert contact surface.
 11. The method of claim 1, inwhich the providing step (b) comprises steps of: (b1) constructing adisc drive including the first actuator and a disc stack having at leastone data storage disc; and (b2) positioning the first actuator so thatthe first transducer is adjacent the disc stack.
 12. The method of claim1, further including a step (g) of detecting many positions of each ofseveral subsequent actuators by repeating a series of steps comprising:(g1) bringing the inert contact surface(s) to bear against a selectedone of the subsequent actuators; (g2) re-using the position sensor manytimes; and (g3) separating the inert contact surface(s) from theselected actuator.
 13. The method of claim 1, further including a step(g) of detecting a respective position of each of hundreds of subsequentactuators by repeating a series of steps comprising: (g1) re-using theposition sensor while urging the inert contact surfaces against aselected one of the subsequent actuators; and (g2) separating the inertcontact surface(s) from the selected actuator.
 14. An apparatus fordetecting a position of a transducer supported by an arm of an actuator,the arm having an arm axis passing therethrough substantially along alength of the arm, the apparatus comprising: a position sensor; andreusable engagement means for temporarily supporting the position sensorin a fixed position relative to the actuator while exerting a near-zeronet torque about the arm axis.
 15. The apparatus of claim 14, in whichthe engagement means bears against the actuator in a directionsubstantially aligned with an axis of rotation about which the actuatoris configured to rotate.
 16. The apparatus of claim 14, in which theengagement means engages the actuator on an end surface thereof.
 17. Theapparatus of claim 14, in which the engagement means is configured toengage at most one head-disc assembly (HDA) at a time, and in which theHDA includes the actuator.
 18. The apparatus of claim 14, in which themultiple use engagement means is a single-piece rigid element.
 19. Theapparatus of claim 14, in which the engagement means can operate withoutmaking a contact with any protrusion of the actuator.
 20. The apparatusof claim 14, in which the actuator is rotatable about a spindle axis,and in which the engagement means can exert a force >1 Newton tocompress the actuator along the spindle axis via several inert contactsurfaces distributed about the spindle axis.
 21. The apparatus of claim14, in which the actuator is rotatable about a spindle axis, and inwhich the engagement means comprises several inert contact surfaces thatcan each exert a respective force upon the actuator, each respectiveforce having a respective component along the spindle axis.
 22. Theapparatus of claim 14, in which the actuator is rotatable about aspindle axis, and in which the engagement means comprises several inertcontact surfaces that can each exert a respective force upon theactuator, each of the respective forces having a respective componentalong the spindle axis greater than 0.1 Newtons.
 23. The apparatus ofclaim 14, in which the engagement means is configured to engage aninterior of a hole in the actuator.
 24. The apparatus of claim 14, inwhich the engagement means is configured to engage a tapered portion ofa hole in the actuator.
 25. The apparatus of claim 14, in which theposition sensor includes a retroreflector.
 26. The apparatus of claim14, in which the circuit includes an optical sensor.
 27. The apparatusof claim 14, in which the engagement means comprises a gas bearing. 28.The apparatus of claim 14, in which the engagement means includes anelastic deformation element clamping the position sensor in the fixedposition relative to the arm.
 29. The apparatus of claim 14, in whichthe engagement means consists of several inert contact surfaces that canbe clamped simultaneously each against a respective predeterminedportion of the actuator.
 30. The apparatus of claim 14, in which theengagement means bears against the actuator in a direction substantiallyaligned with a spindle axis, in which the engagement means engages theactuator on an end surface thereof, in which the engagement means isconfigured to engage at most one head-disc assembly (HDA) at a time, inwhich the HDA includes the actuator, in which the actuator is rotatableabout a spindle axis, in which the engagement means can exert a force >1Newton to compress the actuator along the spindle axis via several inertcontact surfaces distributed about the spindle axis, in which theengagement means comprises several inert contact surfaces that can eachexert a respective force upon the actuator, each respective force havinga respective component along the spindle axis greater than 0.1 Newtons,in which the engagement means is configured to engage a tapered poritonof an interior of a hole in the actuator, in which the position sensorincludes a retroreflector, and in which the engagement means comprisesan air bearing.